1. Field of the Invention
This invention relates generally to methods and apparatus for monitoring parameters associated with the circulatory system of a living subject, and specifically to the non-invasive monitoring of arterial blood pressure.
2. Description of Related Technology
The accurate, continuous, non-invasive measurement of blood pressure has long been sought by medical science. The availability of such measurement techniques would allow the caregiver to continuously monitor a subject""s blood pressure accurately and in repeatable fashion without the use of invasive arterial catheters (commonly known as xe2x80x9cA-linesxe2x80x9d) in any number of settings including, for example, surgical operating rooms where continuous, accurate indications of true blood pressure are often essential.
Several well known techniques have heretofore been used to non-invasively monitor a subject""s arterial blood pressure waveform, namely, auscultation, oscillometry, and tonometry. Both the auscultation and oscillometry techniques use a standard inflatable arm cuff that occludes the subject""s brachial artery. The auscultatory technique determines the subject""s systolic and diastolic pressures by monitoring certain Korotkoff sounds that occur as the cuff is slowly deflated. The oscillometric technique, on the other hand, determines these pressures, as well as the subject""s mean pressure, by measuring actual pressure changes that occur in the cuff as the cuff is deflated. Both techniques determine pressure values only intermittently, because of the need to alternately inflate and deflate the cuff, and they cannot replicate the subject""s actual blood pressure waveform. Thus, true continuous, beat-to-beat blood pressure monitoring cannot be achieved using these techniques.
Occlusive cuff instruments of the kind described briefly above have generally been somewhat effective in sensing long-term trends in a subject""s blood pressure. However, such instruments generally have been ineffective in sensing short-term blood pressure variations, which are of critical importance in many medical applications, including surgery.
The technique of arterial tonometry is also well known in the medical arts. According to the theory of arterial tonometry, the pressure in a superficial artery with sufficient bony support, such as the radial artery, may be accurately recorded during an applanation sweep when the transmural pressure equals zero. The term xe2x80x9capplanationxe2x80x9d refers to the process of varying the pressure applied to the artery. An applanation sweep refers to a time period during which pressure over the artery is varied from overcompression to undercompression or vice versa. At the onset of a decreasing applanation sweep, the artery is overcompressed into a xe2x80x9cdog bonexe2x80x9d shape, so that pressure pulses are not recorded. At the end of the sweep, the artery is undercompressed, so that minimum amplitude pressure pulses are recorded. Within the sweep, it is assumed that an applanation occurs during which the arterial wall tension is parallel to the tonometer surface. Here, the arterial pressure is perpendicular to the surface and is the only stress detected by the tonometer sensor. At this pressure, it is assumed that the maximum peak-to-peak amplitude (the xe2x80x9cmaximum pulsatilexe2x80x9d) pressure obtained corresponds to zero transmural pressure.
One prior art device for implementing the tonometry technique includes a rigid array of miniature pressure transducers that is applied against the tissue overlying a peripheral artery, e.g., the radial artery. The transducers each directly sense the mechanical forces in the underlying subject tissue, and each is sized to cover only a fraction of the underlying artery. The array is urged against the tissue, to applanate the underlying artery and thereby cause beat-to-beat pressure variations within the artery to be coupled through the tissue to at least some of the transducers. An array of different transducers is used to ensure that at least one transducer is always over the artery, regardless of array position on the subject. This type of tonometer, however, is subject to several drawbacks. First, the array of discrete transducers generally is not anatomically compatible with the continuous contours of the subject""s tissue overlying the artery being sensed. This has historically led to inaccuracies in the resulting transducer signals. In addition, in some cases, this incompatibility can cause tissue injury and nerve damage and can restrict blood flow to distal tissue.
Other prior art techniques have sought to more accurately place a single tonometric sensor laterally above the artery, thereby more completely coupling the sensor to the pressure variations within the artery. However, such systems may place the sensor at a location where it is geometrically xe2x80x9ccenteredxe2x80x9d but not optimally positioned for signal coupling, and further typically require comparatively frequent re-calibration or repositioning due to movement of the subject during measurement.
Tonometry systems are also commonly quite sensitive to the orientation of the pressure transducer on the subject being monitored. Specifically, such systems show a degradation in accuracy when the angular relationship between the transducer and the artery is varied from an xe2x80x9coptimalxe2x80x9d incidence angle. This is an important consideration, since no two measurements are likely to have the device placed or maintained at precisely the same angle with respect to the artery. Many of the foregoing approaches similarly suffer from not being able to maintain a constant angular relationship with the artery regardless of lateral position, due in many cases to positioning mechanisms which are not adapted to account for the anatomic features of the subject, such as curvature of the wrist surface.
Another significant drawback to arterial tonometry systems in general is their inability to continuously monitor and adjust the level of arterial wall compression to an optimum level. Generally, optimization of arterial wall compression has been achieved only by periodic recalibration. This has required an interruption of the subject monitoring function, which sometimes can occur during critical periods. This disability severely limits acceptance of tonometers in the clinical environment.
One of the most significant limitations of prior art tonometry approaches relates to incomplete pressure pulse transfer from the interior of the blood vessel to the point of measurement on the surface of the skin above the blood vessel. Specifically, even when the optimum level of arterial compression is achieved, there is incomplete and often times complex coupling of the arterial blood pressure through the vessel wall and through the tissue to the surface of the skin, such that the magnitude of pressure variations actually occurring within the blood vessel is somewhat different than that measured by a tonometric sensor (pressure transducer) placed on the skin. Hence, any pressure signal or waveform measured at the skin necessarily differs from the true pressure within the artery. Modeling the physical response of the arterial wall, tissue, musculature, tendons, bone, skin of the wrist is no small feat, and inherently includes uncertainties and anomalies for each separate individual. These uncertainties and anomalies introduce unpredictable error into any measurement of blood pressure made via a tonometric sensor. FIGS. 1 and 2 illustrate the cross-section of a typical human wrist, illustrating the various components and their relationships during normal (uncompressed) and applanated (compressed) states.
FIG. 3 graphically illustrates the foregoing principles, specifically the variability in the tonometric measurements relative to the invasive xe2x80x9cA-linexe2x80x9d or true arterial pressure. FIG. 3 shows exemplary tonometric pulse pressure (i.e., systolic minus diastolic pressure) data obtained during applanation of the subject""s radial artery to the mean pressure. FIG. 3 demonstrates the differences between the pulse pressures measured with the non-invasive prior art tonometric apparatus and the invasive A-Line catheter; note that these differences are generally neither constant nor related to the actual pulse pressure. Hence, there can often be very significant variance in the tonometrically-derived measurements relative to the invasive catheter pressure, such variance not being adequately addressed by prior art techniques.
Based on the foregoing, there is needed an improved methodology and apparatus for accurately, continuously, and non-invasively measuring blood pressure within a living subject. Such improved methodology and apparatus would ideally allow for continuous tonometric measurement of blood pressure which is reflective of true intra-arterial (catheter) pressure, while also providing robustness and repeatability under varying patient physiology and environmental conditions. Such method and apparatus would also be easily utilized by both trained medical personnel and untrained individuals, thereby allowing certain subjects to accurately and reliably conduct self-monitoring.
The present invention satisfies the aforementioned needs by an improved method and apparatus for non-invasively and continuously assessing hemodynamic properties, including arterial blood pressure, within a living subject.
In a first aspect of the invention, an improved method of obtaining a pressure signal obtained from a blood vessel of a living subject using parametric scaling is disclosed. The method generally comprises applanating a portion of tissue proximate to a blood vessel to achieve a desired condition, and measuring the pressure associated with the blood vessel non-invasively. The measured pressure may then be optionally scaled using parametric data obtained from the subject (or other subjects, for example, on a statistical basis). In one exemplary embodiment of the method, the portion of the tissue (e.g., that proximate to and effectively surrounding the blood vessel of interest) is applanated or compressed to a level which correlates generally to the maximum pulse pressure amplitude for the blood vessel. This greatly minimizes the error between the true intra-vessel pressure and the tonometric reading. The tonometric reading is then optionally scaled (adjusted) for any remaining error based on parametric data comprising the body mass index (BMI) and pulse pressure (PP) for the subject being evaluated. In certain cases, such as those where there is little error or transfer loss resulting from the tissue interposed between the blood vessel wall and tonometric transducer, little or no scaling is needed. In other cases (e.g., where the transfer loss is significant), scaling of the tonometric pressure reading may be appropriate. In one exemplary variant of the method, discrete ranges of parametric data (e.g., BMI/PP) are established such that a given range of data correlates to a unitary (or deterministic) scaling factor or set of factors.
In another exemplary embodiment, a ratio of the BMI to wrist circumference (WC) is formed, and appropriate scaling applied based thereon.
In a second aspect of the invention, an improved apparatus for applanating tissue to provide non-invasive blood pressure measurements is disclosed. The apparatus comprises an applanation element adapted to apply a level of applanation or compression to the tissue proximate to the blood vessel while also measuring pressure tonometrically. In one exemplary embodiment, the applanation element comprises a substantially rectangular pad having an aperture centrally located therein. The aperture is a cylindrical shape having one or more pressure transducers disposed therein and set to a predetermined depth with respect to the contact surface of the pad. A drive mechanism is connected to the element to allow varying levels of force to be applied to the tissue. One or more stepper motors with position encoders are employed to permit precise positioning of the applanation element with respect to the blood vessel/tissue.
In a third aspect of the invention, an improved method for locating the optimal applanation for measuring a hemodynamic parameter is disclosed. The method generally comprises varying the position of the aforementioned applanation element relative to the blood vessel such that varying hemodynamic conditions within the blood vessel are created over time. The optimal level of applanation for the element is then determined by analyzing data obtained tonometrically from the blood vessel (i.e., the overlying tissue), the optimal level subsequently being established to monitor the selected parameter. In one exemplary embodiment, the hemodynamic parameter comprises arterial blood pressure, and the applanation element is varied in position with respect to the blood vessel so as to create a progressively increasing level of compression (so-called xe2x80x9capplanation sweepxe2x80x9d). The optimal applanation occurs where the highest or maximum pulse pressure is observed. An algorithm is used to iteratively analyze the pressure waveform obtained during the sweep and identify the optimum (maximum pulse pressure) point. The applanation level is then adjusted or xe2x80x9cservoedxe2x80x9d around that maximal point, where additional measurement and processing occurs. Optionally, the foregoing methodology may be coupled with optimization routines and positional variations associated with one or more other dimensions (e.g., lateral, proximal, and angle of incidence with respect to the normal, for the human radial artery), such that all parameters are optimized, thereby providing the most accurate tonometric reading.
In a fourth aspect of the invention, an improved method for scaling the blood pressure measurements obtained from a living subject is disclosed. The method generally comprises: determining at least one physiologic parameter of the subject; forming a relationship between the at least one parameter and a scaling function; and using the scaling function to scale raw (i.e., unscaled) blood pressure data. In one exemplary embodiment, the blood pressure measurements are obtained from the radial artery of the subject, and two physiologic parameters are utilized: the first parameter comprises the body mass index (BMI) of the subject, and the second parameter the tonometrically measured pulse pressure (PP). An index or ratio of the BMI to the PP is then formed. This index is compared to a predetermined set of criteria relating the index value to the required scaling factor to be applied to the raw blood pressure data. The scaling criteria may be either discrete (e.g., multiple index xe2x80x9cbandsxe2x80x9d having a different scaling factor associated therewith) or continuous in nature. The required scaling can be accomplished automatically (such as via a look-up table, algorithm or similar mechanism in the system software), or alternatively manually, such as via a nomograph, graph, or table.
In a second embodiment, the BMI is related to the wrist circumference of the subject as determined from the subject. In yet another embodiment, the body fat content of the subject is used to develop a scaling function.
In a fifth aspect of the invention, an improved computer program for implementing the aforementioned methods is disclosed. In one exemplary embodiment, the computer program comprises an object code representation of a C++ source code listing, the object code representation being disposed in the program memory or similar storage device of a microcomputer system. The program is adapted to run on the microprocessor of the microcomputer system. One or more subroutines for implementing the applanation optimization and scaling methodologies described above are included within the program. In a second exemplary embodiment, the computer program comprises an instruction set disposed within the storage device (such as the embedded program memory) of a digital processor.
In a sixth aspect of the invention, an improved non-invasive system for assessing one or more hemodynamic parameters is disclosed. The system includes the aforementioned applanation apparatus, along with a digital processor and storage device. In one exemplary embodiment, the apparatus comprises a pressure transducer disposed in the applanation element which is used to applanate the radial artery of a human. The processor is operatively connected to the pressure transducer and applanation apparatus, and facilitates processing signals from the pressure transducer during blood pressure measurement, as well as control of the applanation mechanism (via a microcontroller). The processor further includes a program memory (such as an embedded flash memory) with the aforementioned algorithm stored therein in the form of a computer program. The storage device is also coupled to the processor, and allows for storage of data generated by the pressure transducer and/or processor during operation. In one exemplary variant, the apparatus further includes a second storage device (e.g., EEPROM) which is associated with the transducer and removably coupled to the apparatus, such that the transducer and EEPROM may be easily swapped out by the user. The removable transducer/EEPROM assembly is pre-configured with given scaling data which is particularly adapted for subjects having certain parametrics (e.g., BMI within a certain range). In this fashion, the user simply evaluates the parametrics, and selects the appropriate assembly for use with the apparatus. The apparatus supplies an appropriate value of PP (e.g., a xe2x80x9ccorrectedxe2x80x9d value derived from recently obtained data), thereby generating the BMI/PP ratio needed to enter the scaling function (e.g., lookup table). Once the appropriate scaling factor is selected, it is automatically applied to the unscaled pressure waveform. No other calibration or scaling is required, thereby substantially simplifying operation of the apparatus while allowing for highly accurate and continuous pressure readings.
In another exemplary variant, the second storage device is configured so as to carry a plurality of scaling factors/functions, the appropriate one(s) of which is/are selected at time of use through parametric data supplied to the apparatus.
In an seventh aspect of the invention, an improved method of providing treatment to a subject using the aforementioned methodologies is disclosed. The method generally comprises the steps of: selecting a blood vessel of the subject useful for measuring pressure data; applanating the blood vessel to an optimal level; measuring the pressure data when the blood vessel is optimally applanated; scaling the measured pressure data; and providing treatment to the subject based on this scaled pressure data. In one exemplary embodiment, the blood vessel comprises the radial artery of the human being, and the aforementioned methods of optimally applanating the blood vessel and scaling the pressure waveform using BMI/PP are utilized.
These and other features of the invention will become apparent from the following description of the invention, taken in conjunction with the accompanying drawings.